The invention relates to an apparatus and a method for processing plastic material.
Devices for processing and pre-treatment of plastic materials are known, for example, from EP 390 873. Such devices generally operate in satisfactory fashion, but it has been shown that in some cases the plastic material carried off via the worm gear is not sufficiently homogeneous, especially in regard to the obtained extent of drying of such plastic materials which must already be fully dry before plastification, for example polyester, to avoid decomposition processes. Thicker foils in addition require expense in drying that goes up as the thickness increases, owing to which, for such goods, separate drying processes such as with dehydrated air, are necessary in special dryers. These dryers operate in a temperature range for which only crystallized goods are permitted; amorphous goods would become sticky and agglomerate. This means that a crystallization process must precede the drying process. But if the goods to be processed are subjected to lengthy treatment in the container by the tool, then with the device in continuous operation, a danger arises that individual plastic particles are caught very early by the removing worm gear, while other plastic particles are caught very late. The plastic particles caught early may still be relatively cold and therefore may not have been given sufficient pre-treatment, dried, crystallized or softened, possibly resulting in inhomogeneities in the material fed through the worm gear to the attached tool, such as an extruder.
To solve this problem, mechanisms have been created such as are known from AT 396 900 B. Through such devices, the homogeneity of the material can be improved. Two or more containers are situated in a series and the plastic material to be processed runs through these containers in turn. In the first container, already pre-comminuted, pre-heated, pre-dried and pre-densified, and thus pre-homogenized material is generated, which is fed to the following container. By this means it is ensured that no untreated, i.e. cold, uncompacted, uncomminuted or inhomogeneous material passes directly to the removal worm gear or to the extruder. It is true that such devices with multiple containers are bulky and take up much space. Also, the design expense, especially in the linking of the containers, is considerable.
With all of the treatment procedures, it must always be kept in mind that plastic amounts either not treated or inadequately treated form inhomogeneous plastic nests in the worm gear, which is detrimental to the quality of the end product. If therefore one desires to obtain end products, whether they be granulates or items extruded into shapes, with the desired quality retained, then the worm gear that transports the inadequately prepared material out of the receptacle must bring the entirety of the material fed by it at the worm gear outlet to the desired quality and temperature, to be able to extrude the material with the desired homogeneity. This initial temperature must be kept relatively high in order to assure that all the plastic particles are sufficiently plasticized. This in turn entails high energy expense and in addition that thermal damage to the plastic material, such as breakdown of the molecular chain length, is to be feared due to the relatively high initial temperature.
In addition, from prior art from AT 407 970 B, a mechanism is known in which the material to be processed is processed continuously in the same receptacle by means of two sets of tools situated one above the other in two successive stages. In the first, by means of the stage carried out of the upper tool set, the material is pre-comminuted and/or preheated and/or pre-dried and/or premixed. In the second stage carried out by means of the lower tool set, the material obtains the same treatment, but less intensively. Material is exchanged between the first and second stage via a permanently open annular gap that is formed between the container wall and a carrier disk. It is true that owing to the friction of the material occurring there between the container wall and carrier disk, the annular gap is not advantageous and not able to be comminuted at will. In addition, the size of the annular gap cannot be altered. With larger containers of this design, the overall open area between the stages is larger than necessary, which leads to an expanded dwell time spectrum of the material.